The test itself is a cheek swab or a blood sample, sent to a laboratory, returning one of three results: normal, carrier, or affected. It never needs repeating. For a breed where a substantial share of the population carries the mutation, that is arguably the highest-value few dollars an owner will ever spend on a dog.
What the result changes is the conversation before anesthesia. A sterilization, a dental, a biopsy, an emergency surgery: all of them involve drug choices, and an affected dog needs those choices made differently, with dose adjustments and alternative agents. A veterinarian who knows the status in advance plans around it as a matter of routine. A veterinarian who finds out afterwards is managing a crisis. The paperwork should be in the chart and, ideally, on the dog's tag or collar, because the practice most likely to need it in a hurry is the emergency hospital that has never met you.
The local advantage is real but should not be relied on blindly. Washington's veterinary community is unusually alert to this, and Seattle has a dense population of herding breeds to keep that awareness sharp. Even so, the practice on the other side of a bad night may be staffed by someone who trained elsewhere, and the responsibility for the information travelling with the dog is the owner's. Carrier status is worth understanding too: a carrier is generally clinically unaffected but should be flagged, and matters greatly if the dog is ever bred.